(1) Field of the Invention
The present invention relates to a fixing device for fixing an unfixed image formed on a recording sheet onto the recording sheet by applying heat to the unfixed image, and relates to an image formation apparatus provided with the fixing device.
(2) Related Art
In electrophotographic image formation apparatuses such as printers and copiers, usually, a toner image corresponding to image data is transferred onto a recording sheet, such as a sheet of recording paper and an OHP sheet, and then a fixing device fixes the toner image transferred onto the recording sheet. The fixing device fixes the toner image onto the recording sheet by applying heat to the toner image on the recording sheet and pressure to the recording sheet.
Patent Literature 1 (Japanese Patent Application Publication No. 2008-40097) discloses a fixing device provided with a heater (heating sheet) which includes an insulating substrate and a resistance heating element disposed thereon. The resistance heating element has a positive temperature coefficient of resistance. In this fixing device, a heating roller is provided so as to face the heater, and a fixing film having a strip-like shape is provided so as to be movable between the heater and the heating roller. The recording sheet passes through a fixing nip between the fixing film and the heating roller.
The resistance heating element has a positive temperature coefficient of resistance, and usually, the resistance of the resistance heating element gradually decreases until its temperature reaches a certain point. When the temperature reaches a point near the Curie temperature (Curie point), its electrical resistance rises sharply. In consequence, current supply to the resistance heating element will be reduced.
Patent Literature 2 (Japanese Patent Application Publication No. 2009-244595) discloses a fixing device provided with a heating sheet as a heater, which includes an insulating substrate and a resistance heating element disposed thereon. The resistance heating element of the heating sheet is divided into portions in the longitudinal direction. The portions of the resistance heating element are connected in parallel, and a PTC element is serially connected to each resistance heating element. Note that PTC elements are characterized in that the resistance increases when the temperature exceeds a certain point (Curie temperature). In each serial connection, current is independently supplied to the portion of resistance heating element and the PTC element.
With the structures of Patent Literatures 1 and 2, the fixing device is provided with the resistance heating element as described above. Hence, unlike conventional fixing devices using a halogen lamp or the like as a heater, the fixing device needs not to control the fixing temperature by using a temperature detection element. Hence, the fixing device needs not to be provided with a controller for controlling a halogen lamp or the like as a heater. Therefore, it is unnecessary to use an expensive temperature detection element such as a thermopile, and the fixing device is economical. Furthermore, since the controller is unnecessary, there is no risk that runaway occurs in the control unit due to software bugs or electrical noise and the temperature of the heater reaches a dangerous level in a short period.
However, since the heaters (heating sheets) used in Patent Literatures 1 and 2 have a strip-like structure in which a thick-film resistance heating element is provided on the insulating substrate, there is a risk for the occurrence of breakage such as a crack in the insulating substrate due to physical vibration to the heater (heating sheet) or thermal stress caused at a high temperature equal to or higher than 150° C. A crack in the insulating substrate would lead to the occurrence of a rupture in the resistance heating element on the insulating substrate, and would make it impossible to supply power to the resistance heating element.
If it is impossible to supply power to the resistance heating element, it is impossible to cause the resistance heating element to generate heat. This causes, for example, a fixing failure, which is a problem that a toner image on the recording sheet can not be surely fixed on the recording sheet.
In addition, since the structures disclosed in Patent Literatures 1 and 2 do not include any temperature detection element such as a thermistor and a thermopile, the devices can not detect the abnormality that the resistance heating element is not supplied with power and the resistance heating element does not generate heat. However, if a temperature detection element such as a thermistor and a thermopile is adopted to monitor the temperature of the heater (heating sheet) and detect an abnormality in the resistance heating element, the cost efficiency will be compromised.
In particular, in the case of the structure disclosed in Patent Literature 2 for example, where each portion of the resistance heating element is separately supplied with power, it is necessary to monitor the temperature of each portion of the resistance heating element in order to detect an abnormality in each resistance heating element. If many temperature detection elements are adopted for monitoring the temperature of each portion of the resistance heating element, this significantly compromises the cost efficiency.